I’ve seen this error twice, yesterday and today, when I started filling in a form on Mantis (either a new issue or a response), and waited too long to submit. It happened again just now, after maybe 30 min. Reloading the page allows me to re-enter/copy the content and proceed, but this limitation seems quite unfriendly to bug reporters, who are (ideally!) collecting and confirming information over a period of time while filling in, before actually submitting.
APPLICATION ERROR #2800 Invalid form security token.
Google takes me here which says:
Unfortunately, this problem cannot be fixed without a major rework of the way sessions and form security are handled in MantisBT.